The present disclosure relates to an optical unit that forms an electrostatic image by scanning, in a main scanning direction of an image bearing member, light emitted from a light source, and to a light scanning device and an image forming apparatus in which the optical unit is used.
A light scanning device used in an image forming apparatus of an electrophotographic system includes a light source and a polygon mirror. A laser beam emitted from the light source is reflected by a polygon mirror driven to rotate. The laser beam scans the circumferential surface of a photosensitive drum in a main scanning direction, whereby an electrostatic latent image is formed on the photosensitive drum.
On mirror surfaces of the polygon mirror, fluctuation in flatness is sometimes caused by a bulge due to a molding condition, a centrifugal force of rotation, or the like. If the flatness is different depending on the mirror surfaces, the magnification in the main scanning direction of a laser spot condensed on the circumferential surface of the photosensitive drum is different for each of the mirror surfaces. As a result, an irradiation position of a dot shifts in every scanning. A jitter appears in an image at a cycle of one revolution of the polygon mirror. In particular, in a halftone image on a screen or the like, dots markedly interfere with one another and a jitter tends to occur, causing deterioration in image quality.
Conventionally, there is known a method of detecting one revolution of the polygon mirror with a reflection optical sensor and the like and correcting emission timing and magnification of a laser beam when the mirror surfaces are used. However, in this method, since the dedicated reflection optical sensor is used to specify the mirror surfaces, an increase in costs due to the addition of the sensor is inevitable.
It is an object of the present disclosure to provide an optical unit that can easily specify mirror surfaces of a polygon mirror without using a dedicated sensor and a light scanning device and an image forming apparatus in which the optical unit is used.